Transcript Electronic and Computer Systems Service

Electronic and Computer
Systems Service
Chapter 46
© 2012 Delmar, Cengage Learning
Objectives
• Diagnose related engine and electrical
problems prior to computer repair
• Describe the theory and operation of on-board
diagnostics
• Read trouble codes
• Use a scan tool
• Confirm closed loop
• Test sensors and actuators
• Diagnose computer wiring problems
• Diagnose and replace a computer
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Introduction
• Computer systems have become sophisticated
– Necessary to consult model-specific service
information
• Step-by-step procedures for troubleshooting
– Today's technicians must be able to read a
wiring schematic
• In order to diagnose and repair modern
computerized vehicles
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Inspection Sequence
• Computers have self-diagnostic ability
– Logical diagnosis sequence must be followed
before checking computer
• Digital multimeter
– Used to measure electricity in electronic circuits
• Visual inspection
– Can often determine problem cause
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Perform Diagnostic Tests
• Analyze the cause of the problem rather than
just fixing the problem’s result
– Listen during cranking for an even rhythm and
then for a smooth idle
– Check base timing setting on engines with a
distributor ignition
– Do a charging system test before beginning a
diagnostic procedure
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On-Board Diagnostics
• Computers detect incorrect electrical conditions
– Save trouble codes to memory
• Key is turned on: computer does a self-check of
its circuits
• Diagnostic tree
– Provides a step-by-step diagnostic procedure
• Sensors
– Cause electronic control problems more often
than actuators
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Reading Trouble Codes
• Different ways to read trouble codes
– OBD II systems have standardized connectors
and procedures
– Most systems have a diagnostic link connector
(DLC)
• Scan tool can be connected to it to read codes
• Procedure for retrieving fault codes varies
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Scan Tools
• Portable computer
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Reads data from the on-board computer
Have specific software cartridges
Handheld and can be taken on a road test
Limited to diagnosing computer problems
• Communication between scan tool and
computer
– Unidirectional or bidirectional
• Parameter identification data
– Included in on-board diagnostics
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Breakout Box
• Diagnoses
problems
– Probes inserted
into pin holes
access sensors
and actuators
– Reads raw
system values
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Retrieving Trouble Codes
• OBD I
– Scan tools were not so widely owned by
automotive technicians
• OBD II scan tools
– Powered through the DLC
– Do not disconnect or connect while the key is on
– Remove before removing connections to
electronic components
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Working with Codes
• More than one code is given
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Fix the lower number code first
Fix the problem first and then start again
Be sure to check power and grounds
Erase codes and test drive the car to reset codes
• OBD II cars
– Scan tool must be used to read codes
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Erasing Trouble Codes
• Code can remain in memory even though a
problem has been corrected
– Clear codes after the repair has been made
• Older vehicles: procedure that shuts off power to
computer can be used to erase codes
• OBD II vehicles: scan tool command required
• Manufacturer's methods vary
• Scan tool
– Erases codes without disconnecting anything
– Test drive car to see if any codes return
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Scan Tool Snapshot
• Scan tool has a feature like an airplane flight
recorder
– Helps catch glitches and intermittent problems
• Settings
– Snapshot: series of pictures representing the
conditions present when the DTC was set
– Automatically record when any fault code occurs
during the test drive
– Freeze frame
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Closed Loop
• Computers require correct inputs from sensors
and correct actions from actuators
– Several things must occur for a computer system
to go into closed loop
• Methods for confirming closed loop vary
– Examples: DMM, scan tool, and a lab scope
– Do not use ohmmeter to test an oxygen sensor
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Fuel Trim Diagnosis
• Diagnosing fuel trim with scan tool
– Drive vehicle under same conditions where
problem occurred
– Restricted fuel filter or low fuel pump output
increase fuel trim under load
– Leaks that allow air into intake system result in
higher fuel trim values
– Plugged or sticking fuel injector affects fuel trim
cells equally as rpm increases
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Digital Waveforms
• Waveform
– Better diagnostic tool than digital volt-ohmmeter
• Tools capable of displaying voltage or frequency
in waveform
– Digital storage oscilloscopes
– Graphing multimeters
– Four channel scopes
• Can display four waveforms
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Logic Probe/Power Probe
• Three colored LEDs instead of one bulb
– Touching probe to ground lights green LED
– Red LED illuminates when touched to a power
source
– Yellow light comes on when a pulsed voltage is
sensed
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Sensor and Actuator Testing
• Sensor testing strategy
– No-start condition occurs if a distributor reference
or crankshaft sensor signal is lost
– Defective or misadjusted TPS can send an
excessive voltage to computer as WOT
– With the key on, do not disconnect any electrical
components unless instruction says to do this
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Diagnosing Sensor Problems
• When using a scan tool to diagnose sensors
and actuators, use the following sequence:
– Check input sensors
– Perform a quick check of input switches
– Check outputs
• Sensor types
– High authority sensors
– Passive sensors
– Active sensors
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Sensor Tests
• Vehicle speed sensor
– Supplies input for electronic speedometers and
cruise control systems
• Also controls torque converter clutch
• Types of speed sensors
– Photoelectric and magnetic AC generator
• Failed sensor can cause:
– Premature or no converter clutch lockup
– Lack of change in steering assist
– Inoperative cruise control and speedometer
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Oxygen Sensor
• Enriches mixture so reduction catalyst can work
– Must provide a lean enough mixture for
hydrocarbons and carbon monoxide to oxidize
• Characteristics
– Start-up varies
– Lazy sensor produces voltage slowly and does
not change back and forth
– Range is tested by creating full rich and full lean
conditions
– Wide range oxygen sensors can accurately
detect air-fuel ratios over a wide range
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Load Sensors
• Include MAP, vacuum, and MAF
– Tell computer how much air is entering engine
– Affect ignition timing and air-fuel ratios
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MAP Sensor and BARO Sensors
• MAP sensor
– Basic fuel delivery to the engine is determined by
MAP sensor, CKP, and ECT
– Engine load is high: fuel injectors are on longer
– Vacuum higher: MAP sensor voltage drops
• BARO sensors
– Monitor changes in weather or altitude
– Several different types
– Defective sensor causes poor high-altitude
performance or spark knock
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Vacuum Sensors
• Measure difference between atmospheric
pressure and intake manifold pressure
– Systems that use vacuum sensors must also use
BARO sensors
– BMAP is a combination barometric and MAP
sensor
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Throttle Position Sensor
• Potentiometer mounted on throttle shaft
– Defective or misadjusted TPS causes hesitation
when accelerating
• Check with voltmeter or ohmmeter
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Coolant Temperature Sensor
and Air Temperature Sensors
• Coolant temperature sensor
– Affects how the engine operates in all conditions
– Common problem: computer system will not go
into closed loop when engine is warm
– Use ohmmeter or voltmeter to test
• Air temperature sensors
– IAT sensor works like a coolant temperature
sensor
• Fine tunes air-fuel mixture
• Compensates for air density
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Airflow Sensor Service
• Fuel systems controlled by an airflow sensor
– React poorly to vacuum leaks
– Dirt causes problems in vane airflow sensor
– Intake manifold popback causes the door to
bend or break
• MAF sensors have no moving parts
– Hot film MAF sensors produce a variable
frequency instead of voltage
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Knock Sensor Service
• Help prevent engine knock
– Loose bracket or other vibration causes retarded
timing
– Computer senses an inoperative knock sensor:
P0324 through P0334 code will set
– Broken or damaged knock sensor wiring is often
the cause of a knock sensor code
– Test by rapping on the engine near sensor with a
metal tool
– Many engines use a newer style of knock sensor,
called a resonance knock sensor
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Actuator Service
• Actuators include:
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Solenoids
Fuel injectors
Stepper motors
Motors for electronic suspension hydraulic
controls
• Test an actuator
– Done by checking for voltage at actuator control
terminal
– Test according to individual service instructions
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Repair the Problem
• After repairing problem
– Road test vehicle again
• Test drive allows a late-model computer to relearn
its best adjustments
– Use scan tool to erase codes
• After replacement of the computer or when a
battery has been disconnected
– Poor drivability and performance can result until
computer relearns best drivability settings
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Computer Wiring Service
• Common cause of problems in computer
systems
– Poor electrical connections
• Include loose or corroded connections and
grounded wires
• Always use a wiring diagram when working on
computer systems
– Computer must have good power and ground
connections
– Twisted pair wiring carries very small amounts of
current
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Computer Wiring Service
(cont’d.)
• Electronic updates
– Regular occurrence among manufacturers
• Battery voltage must be stable during
reprogramming
– Can take an hour or more to complete
• Computer location
– Usually mounted in driver’s compartment
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Static Electricity
• Static electricity from the front seat is a concern
– People who work around sensitive components
sometimes wear a ground strap
– Touch ground before touching computer
– Do not take the computer out of container until
you are already in the front seat
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Electrical Damage to a Circuit
• Too much electrical current
– Causes heat that damages an electrical circuit
– Damaged connections are usually the reason for
failure
– Semiconductors are designed for only a limited
amount of current
– Bus diagnosis is similar to other electrical system
diagnoses
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning